fix: supersede ALL old reviews, not just the most recent

Previously findOwnReview returned only the single most-recent matching
review, so on PRs with multiple force-pushes only the latest old review
got superseded. The rest accumulated as unsuperseded stale reviews.

Changes:
- Add findAllOwnReviews() to collect all non-superseded matching reviews
- Loop over all old reviews in the supersede phase
- Add GetTimelineReviewCommentIDForReview() to find comment IDs by
  review ID (fetches review body, matches in timeline by prefix)
- Each old review gets independently superseded and its inline comments
  resolved

The old findOwnReview is kept for backward compat (tested, may be
useful as a utility).

cmd/review-bot/main_test.go

@@ -841,3 +841,24 @@ func cleanEnv() []string {
 	}
 	return env
 }
+
+func TestFindAllOwnReviews(t *testing.T) {
+	reviews := []gitea.Review{
+		{ID: 1, Body: "<!-- review-bot:sonnet -->\nfirst review"},
+		{ID: 2, Body: "<!-- review-bot:gpt -->\nother bot"},
+		{ID: 3, Body: "<!-- review-bot:sonnet -->\nsecond review"},
+		{ID: 4, Body: "~~Original review~~\n<!-- review-bot:sonnet -->\nsuperseded"},
+		{ID: 5, Body: "<!-- review-bot:sonnet -->\nthird review"},
+	}
+
+	got := findAllOwnReviews(reviews, "<!-- review-bot:sonnet -->")
+	if len(got) != 3 {
+		t.Fatalf("findAllOwnReviews() returned %d, want 3", len(got))
+	}
+	wantIDs := []int64{1, 3, 5}
+	for i, r := range got {
+		if r.ID != wantIDs[i] {
+			t.Errorf("got[%d].ID = %d, want %d", i, r.ID, wantIDs[i])
+		}
+	}
+}